Control and tracking system and method for a solar power generation system

ABSTRACT

Embodiments of a solar reflector assembly and methods of controlling a solar reflector assembly are generally described herein. Other embodiments may be described and claimed.

RELATED APPLICATION

The present application claims the benefit of the filing date of U.S. Provisional Application Ser. No. 61/525,410, filed Aug. 19, 2011, the entire disclosure of which is incorporated by reference herein.

FIELD

This disclosure generally relates to solar power generation systems, and more particularly, to a control system and method for a solar power generation system.

BACKGROUND

Reflective solar power generation systems may either use a number of spaced apart reflective panels that surround a central tower and reflect sunlight toward the central tower or parabolic-shaped reflective panels that focus sunlight onto a tube at the focal point of the parabola defining the reflective panels. The latter system may be referred to as a solar trough system. With solar trough systems, the structures that support the reflective panels and the tube may deflect due to static loads. Accordingly, instead of focusing sunlight generally along a central axis of the tube, the reflective panels may focus the sunlight at a location that is offset relative to the central axis of the tube. Therefore, the temperature of the heat transfer fluid in the tube may not reach a required or preferred level. The noted deflections due to static loads may be greater for large trough systems. In contrast, when the sunlight is focused onto the tube, the heat transfer fluid may become excessively hot and lose viscosity in trough systems that use large reflectors. The control systems used for solar trough systems may only track the position of the sun without considering the noted deflections due to static loads and/or maintaining the temperature of the heat transfer fluid at a certain level or within a certain range.

SUMMARY

According to one aspect, a method of controlling a solar reflector assembly is disclosed, where the solar reflector assembly may include at least one frame, at least one reflector mounted on the frame, a control system configured to move the frame, and a tube having a central axis and configured to have therein a heat transfer fluid being heated by the reflector focusing sunlight onto a focal line configured to be generally aligned with the central axis, the tube coupled to the frame with at least one tube support. The method includes determining an offset between the focal line and the central axis, and moving the frame to move the central axis toward the focal line, to reduce the offset.

According to another aspect, a solar reflector assembly includes at least one frame, at least one reflector mounted on the frame, a tube having a central axis and configured to have therein a heat transfer fluid being heated by the reflector focusing sunlight onto a focal line configured to be generally aligned with the central axis, the tube coupled to the frame with at least one tube support, and a control system configured to move the frame, the control system comprising a processor and a data storage device. The processor is configured to execute a code stored in the data storage device to determine an offset between the focal line and the central axis, and move the frame to move the central axis toward the focal line to reduce the offset.

According to another aspect, a method of controlling a solar reflector assembly is disclosed. The solar reflector assembly may include at least one frame, at least one reflector mounted on the frame, a control system configured to move the frame, and a tube configured to have therein a heat transfer fluid being heated by the reflector focusing sunlight on the tube, the tube coupled to the frame with at least one tube support. The method includes determining a variable indicative of a temperature of the heat transfer fluid, moving the frame between a focused position wherein sunlight is focused on the tube by the reflector and a defocused position wherein sunlight is less focused on the tube by the reflector than the focused position to control the temperature of the heat transfer fluid.

According to another aspect, a solar reflector assembly includes at least one frame, at least one reflector mounted on the frame, a tube configured to have therein a heat transfer fluid being heated by the reflector focusing sunlight on the tube, the tube coupled to the frame with at least one tube support, and a control system configured to move the frame, the control system comprising a processor and a data storage device. The processor is configured to execute a code stored in the data storage device to determine a variable indicative of a temperature of the heat transfer fluid, and move the frame between a focused position wherein sunlight is focused on the tube by the reflector and a defocused position wherein sunlight is less focused on the tube by the reflector than the focused position to control the temperature of the heat transfer fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a reflector frame assembly according to one exemplary embodiment.

FIG. 2 shows a side view of the reflector frame assembly of FIG. 1.

FIG. 3 shows a tube for carrying a heat transfer fluid in a solar reflector power generation system.

FIG. 4 shows a side view of a frame having a reflector according to one exemplary embodiment.

FIG. 5 shows the tube of FIG. 3 with reflective light rays forming a radiation band thereon.

FIGS. 6-8 show three positions of the frame of FIG. 3.

FIG. 9 shows a side view of the reflector frame assembly of FIG. 1 with static load deflections according to one exemplary embodiment.

FIG. 10 shows a perspective compressed view of a frame with static load deflections according to one exemplary embodiment.

FIG. 11 shows a reflector frame assembly according to one exemplary embodiment.

FIG. 12 shows a control system according to one embodiment.

FIG. 13 shows a method of controlling a reflector frame assembly according to one embodiment.

FIG. 14 shows a bell-shaped curve representing the distribution of reflected light rays striking a tube of the frame assembly of FIG. 1 according to one embodiment.

FIG. 15 shows a curve represent the distribution of reflected light rays striking a tube of the frame assembly of FIG. 1 according to another embodiment.

FIG. 16 shows three bell-shaped curves representing the distribution of reflected light rays striking a tube of the frame assembly of FIG. 1 when the frame is oscillated according to another embodiment.

FIG. 17 shows a method of controlling a reflector frame assembly according to one embodiment.

DETAILED DESCRIPTION

Referring to FIG. 1, a plurality of reflector frame assemblies 100 forming a section of a solar power generation system is shown. Each reflector frame assembly 100 includes a frame 102, which is rotatably mounted on one or more support pylons 104 and can rotate about a center axis or rotation axis 200 to track the daily east to west movement of the sun. Referring also to FIG. 2, each frame 102 has a concave or trough-shaped side, to which one or more reflectors 106 (only one reflector is shown in FIG. 1) are connected. In the embodiments described herein, the reflectors 106 are parabolic. The reflectors 106 may be constructed from any type of rigid (e.g., glass) or flexible material (e.g., reflective film) that provides a reflective surface. The reflectors 106 may be constructed from a flexible reflective material that is mounted to a backing structure. Examples of reflectors and backing structures are described in US 2009/0101195 and US 2011/0094502, the entire disclosures of which are expressly incorporated by reference. The reflectors 106 can be connected to the frame by any device and/or method. An example of a device and method for attaching the reflectors to the frame is described in detail in U.S. patent application Ser. No. 13/491,422, filed Jun. 7, 2012, the entire disclosure of which is expressly incorporated by reference.

Referring to FIGS. 2-5, the reflectors 106 reflect and focus sunlight onto a tube 110, which may extend generally along a focal line 108 of one or more frames 102. A focal line 108 as disclosed is a theoretical concept and may be in the form of a focal band in practice (i.e., having length and width). However, a central axis of such a focal band may be considered a focal line. In the example of FIG. 1, the tube 110 is shown to extend generally along the focal line 108 of four frames 102A-102D. The tube 110 may be mounted with tube mounts 112 to each frame 102. When the reflectors 106 are directly facing the sun, a longitudinal central axis 111 of the tube 110 may be generally coaxial with a focal line 108 of the reflectors 106 (shown in FIG. 3). Accordingly, the reflectors 106 reflect the sunlight generally onto the tube 110 at a focal line 108 which may generally correspond to the longitudinal central axis 111 of the tube 110. The tube 110 serves as a conduit for carrying a heat transfer fluid (HTF) that can transfer the heat generated by the focused sunlight to a power generation section (not shown) of the solar power generation system.

Each reflector frame assembly 100 includes a drive mechanism 113 and controller 114, which may be collectively referred to herein as a control system 115. Each frame 102 is rotated about the axis 200 (shown in FIG. 1) by the control system 115 to track the daily movement of the sun. The direction of rotation is shown by the arrow 202 in FIG. 2. The control system 114 may provide continuous tracking of the sun, thereby providing continuous focusing of sunlight onto the tube 110. Any type of analog and/or digital control system utilizing classical and/or modern control techniques may be used to provide continuous and or discrete solar tracking of the reflector frames 102. An example of a control system and methods for rotating the frame assemblies 100 and solar tracking thereof is provided in U.S. Patent Application Publication No. 2010/0229851, the entire disclosure of which is expressly incorporated by reference.

The reflector 106 (only half of a reflector 106 is shown in FIGS. 2 and 4) has a principal axis 204 and the focal line 108. Light rays 208 that are parallel to the principal axis 204 are focused onto the focal line 108. Because the tube 110 is positioned such that the longitudinal central axis 111 passes through the focal line 108, the light rays 208 striking the reflector 106 are reflected and focused onto the longitudinal central axis 111 of the tube 110 to form the focal line 108. However, the outer surface of the tube 110 is spaced from the longitudinal central axis 111 by a distance defined by the radius of the tube 110. Accordingly, with reference to FIG. 5, the focused light rays 208 form a focal band or radiation band 116 on the surface of the tube 110. Light rays 208 that are slightly offset from parallel or generally offset when striking the reflector 106 may either strike the tube 110 outside the radiation band 116 or completely miss the tube 110. For the HTF in the tube 110 to reach the highest temperature possible considering the contemporaneous physical conditions of the frame assembly and the surrounding environmental conditions, the greatest possible number of light rays 208 must be captured by the reflector 106 and focused onto the tube 110. To do so, the reflector 106 must be oriented so as to be directly facing the sun such that the principal axis 204 theoretically intercepts a center of the Sun. The control system 115 may continuously rotate each frame 102 from east to the west as shown by the three progressive positions of FIGS. 6-8 (i.e., morning, noon and afternoon, respectively) so that the reflector 106 is substantially directly facing the sun.

The control system 115 may utilize tracking algorithms for tracking the location of the sun. Such tracking algorithms use location and the angular position of the frames 102, and the date and time of day to estimate the position of the sun. Each of the frames 102 may include one or more inclinometers, which can provide the control system 115 with the angular positions of the frame 102. By having an estimated position of the sun and the location and position of the frame 102, the control system 115 can rotate the frame 102 to track the movement of the sun. Instead of or in combination with using the solar tracking algorithm, the control system 115 may also utilize thermocouples placed at one or more locations along the tube 110 to measure the temperature of the tube 110 and/or the temperature of the HTF. The control system 115 can then rotate the each or several frames 102 to generally maximize the measured temperature, which may correspond to a generally optimum tracking of the sun. As described in detail below, instead of or in combination with the above tracking devices and methods, the control system 115 may use one or more optical sensors to continuously track the location of the sun.

When a reflector 106 is slightly offset from directly facing the sun, the number of parallel light rays 208 (i.e., parallel to the principal axis 204) is reduced as compared to a scenario where the reflector is directly facing the sun. Accordingly, the number of reflected light rays intercepting the focal line 10 is slightly reduced. Therefore, the intensity of the radiation band 116 is slightly reduced and the HTF in the tube 110 may not reach a possible maximum temperature considering the contemporaneous physical conditions of the frame assembly and the surrounding environmental conditions. When the reflector 106 is even more offset, the intensity of the radiation band 116 is further reduced. Therefore, in order to achieve the highest possible temperature of the HTF in the tube 110, each frame 102, hence the one or more reflectors 106 mounted on each frame, must continuously track the movement of the sun and directly face the sun throughout the day as accurately as possible. As described in detail below, the control 115 may make solar tracking adjustments in order to compensate for static loads, compensate for misalignment, defocus the reflectors to reduce the temperature of the HTF, and/or provide generally uniform heat distribution on the tube 110.

The components of a theoretically rigid solar reflector assembly 100, such as the frame 102, tube mounts 112 and/or the tubes 110 would not deflect under the static loads of the frame members or objects mounted to the frame members or be misaligned relative to each other. Accordingly, the longitudinal central axis of the tube 110 would be coaxial with the focal line 108. However, absolute rigidity cannot be achieved. Furthermore, a highly rigid structure that diminishes any static load deflections to a negligible level may not be practical considering the costs of manufacturing, transportation, assembly, operation and maintenance. Accordingly, the reflector frame assembly 100 may not be constructed with such rigidity so as to relatively eliminate or render negligible the effect of static loads on the frames 102. Large reflector assemblies may also exacerbate such deflection problems. Therefore, the static loads exerted by the frame members and any objects attached to the frame members may deflect the frame members themselves and/or the overall frame 102, thereby affecting the above-described focusing function of the entire frame 102 depending on the angular position of the frames 102 about the rotation axis 200.

Referring to FIG. 9, a frame 102 is shown in a position where the principal axis 204 is generally horizontal. This position may generally correspond to either the morning or late afternoon positions of the frame 102. The position 220 of the tube mounts 112 and the tube 110 shown in FIG. 9 corresponds to a position when the tube mounts 112 (and/or all components of the reflector assembly 100) are rigid. In position 220, the principal axis 204 may generally intersect, the longitudinal central axis 111 of the tube 112. However, as shown in FIG. 9, the tube mounts 112 may be defined by two cantilever beams that can deflect due to the loads exerted thereon by the tube 110 and the HTF carried in the tube 110. Accordingly, as shown by position 222, the weight of the tube 110 including the weight of the HTF may cause the tube mounts 112 to deflect. This deflection is shown in an exaggerated manner in FIG. 9. Depending on the degree of the deflection in the tube mounts 112, some of the light rays reflecting from the reflectors 106 may slightly, significantly, or completely miss the tube 110. Therefore, the deflection in the tube mounts 112 may result in the HTF not reaching a certain temperature.

Referring to FIG. 10, the deflection of the tube mounts 112 may vary along the lengths of the frame assemblies 100. As shown in FIG. 7, in the generally vertical position of the principal axis 204, which may correspond to the noon position of the frame 102, the deflection in the tube mounts 112 may be small, or negligible. Therefore, in the operating range of each frame 102, the deflection of the tube mounts 112 may be highest in the morning and late afternoon positions and diminish, as the frame 102 moves toward the noon position.

As described further below, the control system 115 can rotate the frame assembly 100 to compensate for the deflection in the tube mounts 112. For example, referring to FIG. 9, the control system 114 can rotate the frame assembly 100 in the direction of the arrow 224 in order to move the tube 110 from position 222 toward position 220. Although the tube mounts 112 remain deflected even near position 222, albeit slightly less than when in position 220, the tube 110 will be positioned closer to the tube of position 220, and therefore, intercepting more of the light, rays that are reflected from the reflectors 106 as compared to the tube 110 of position 222. Therefore, with the control system 115 making a compensating, rotation in the direction of the arrow 224, the HTF may reach a preferred temperature. The compensating rotation along the arrow 224 may be highest at the extreme operating positions of the frame 102 (i.e., morning and late afternoon positions) and diminish toward the noon position.

Referring to FIG. 11, a frame 102 is partially shown. At least some of the longitudinal frame members 118 are shown to be deflected or sagging under their own weight. This deflection is shown in an exaggerated manner in FIG. 11. The lateral frame members 120 may also deflect under static loads including the lateral members' own weight. All of the frame members may deflect under their own weight and the loads exerted thereon by the weight of the other frame members. Accordingly, the entire frame 102 may deflect or sag. In the position of the frame assembly 100 shown in FIG. 7, the left and right sides of the frame 102 may deflect or sag nearly symmetrically about the axis of rotation 200. Accordingly, the deflections of the frame members and/or the entire frame 102 may depend on the angular position of the frame 102. Although not shown, various frame members and/or the entire frame 102 may be slightly twisted so as to affect the focusing of the light rays onto the tube 110.

As described in detail below, the control system 115 can rotate a frame 102 or a plurality of connected frames 102A-D to compensate for the deflection and/or twists in the frame members and/or the entire frames 102. For example, the deflection or sagging in the entire frame 102 as shown in FIG. 11 may cause at least a section of the tube 110 to move to a position 222 as shown in FIG. 9. The control system 115 can rotate the frame 102 in the direction of the arrow 224 in order to move the tube 110 from position 222 toward position 220. The deflections in the frame members and/or the entire frame 102 may be measured and/or numerically computed based on the physical and material properties of the frame members and provided to the control system 115.

The frame assembly 100 may be constructed at the operating site of the solar power generation system by onsite assembly of the individual frame members of each frame 102 or onsite assembly of preassembled sections of the frames 102. Due to possible manufacturing inconsistencies, variations and/or defects of a few or some of the parts of the frame assemblies 100, or improper installation of the frame assemblies 100, the frame assemblies 100 may be misaligned when assembled on site. As a result, a few or some of the frames 102 may be misaligned such as to cause slight misalignment in the focusing of the light rays from the reflectors 106 onto the tube 110. An example of such a misalignment is described in detail blow.

Referring back to FIG. 1, the frame assembly 100 is shown as having four frames 102A-102D. All of the frames 102A-D may be rotationally driven about the rotation axis 200 by a single drive mechanism 113 of the control system 115 according to commands from the controller 114 to track the daily movement of the sun. Thus, the frames 102A-D rotate together during solar tracking. The frames 102A-D may be misaligned such that each frame is rotationally offset relative to another frame 102A-D. For example, when the frame 102A is positioned at an angle of 35 degrees relative to a horizontal position, the frame 102B may also be positioned at an angle of 35 degrees. The frames 102C and 102D, however, may be positioned at 36 degrees and 37 degrees, respectively. As a result, if the frames 102A and 102B are correctly positioned for tracking the sun, the frames 102C and 102D are offset from the correct tracking position by 1 and 2 degrees, respectively. The control system 115 can rotate the frames 102A-D to compensate for the misalignment in the frames 102A-102D by rotating the frames 102A-102D to an angle that corresponds to an average of the angles of the frames 102A-102D. In the above example, the control system 115 can position the frames 102A-102D at an angle of 35.75 degrees, which is the average of 35, 35, 36 and 37 degrees.

As described above, the control system 115 can rotate the frame 102 to compensate for any misalignment, which is not limited to the above example, and may include any misalignment between any members and/or sections of each frame assembly 100. The misalignment in the frame assemblies 100 can be measured on site and the corresponding measurement data can be provided to the control system 115.

Referring to FIG. 12, the control system 115 may include a processor 300 and a data storage module 302 as parts of the controller 114. The drive mechanism 113 may be coupled to the processor 300 for receiving commands from the processor to actuate or move one or more of the frames 102 (e.g., four frames 102 as shown in FIG. 1). The drive mechanism 113 may include one or more electric motors and any associated mechanisms such as gearing or other mechanisms that convert rotational motion to linear motion or a combination or rotational and linear motion. The drive mechanism 113 may also include hydraulic actuators such as linear hydraulic actuators and associated mechanisms for converting the linear motion of the hydraulic actuators to rotational motion or a combination of rotational and linear motion. The control system 115 may also include one or more sensors, which are collectively shown as sensor module 306. The sensor module 306 may include any type of sensor that can detect the linear and/or angular position of the frame, the position of the sun, the deflection of one or more frame members (e.g., strain gage), temperature at one or more locations on the tube 110, temperature of the HTF at one or more locations along the tube 110, one or more optical sensors that can measure the intensity and dimensions of the radiation band 116 and/or one or more imaging sensors that capture images of the frames 102 and/or the tube 110. The sensors of the sensor module 306 may be near the processor 300 and/or the data storage module 302. One or more sensors of the sensor module 306 may be remotely located from the processor 300 and/or the data storage module 302. For example, strain gages may be positioned on several frame members to measure the deflections of these frame members during operation of the frame as disclosed. Such strain gages may then transmit data to the processor 300 with wires or wireless communication. The control system 115 may also include other components that may be required for operation of the control system 115 as disclosed such as a power supply or one or more input/output ports (not shown).

Referring to FIG. 13, a method 400 for controlling the frame 102 when compensating for static loads is shown. The method 400 determines an offset between the focal line 108 and the longitudinal central axis 111 of the tube 110 (block 402). The method then moves the frame to move the longitudinal central axis 111 toward the focal line 108 to reduce the offset. The method 400 may determine an actual position of the tube 110 and then move the frame 102 to move the tube 110 from the actual position toward the rigid position of the tube when the actual position is offset relative to the rigid position. The method 400 may determine the shift or offset in the position of one or more sections of the tube 110 relative to the corresponding sections of the focal line 108. The method 400 then rotates the frame 102 to reduce the offset or rotate the frame 102 toward the theoretically rigid position of the frame to compensate for static loads exerted on the frame 102 and the tube mounts 112. The shift or offset in the position of the tube 110 relative to the focal line as described herein may be referring to a shift in the position of a section or the entire tube 110 relative to corresponding section or the entire focal line 108, respectively.

The shift or offset in the position of the tube 110 relative to the focal line 108 may be caused by a deflection in any support structure of the tube, such as deflection in the tube mounts 112, generally the deflection of one or more frame members which may cause the reflectors to shift the focal line 108, and/or any misalignment in the components of the solar reflector assembly 100. For example, a shift in the position of the tube 110 relative to the focal line 108 may be caused by the static loads on the tube mounts 112, the static loads of the tube itself, or the static loads on one or more frame members. A shift in the position of the tube 110 relative to the focal line 108 may be caused by a shift or offset in the reflectors 106 due to static loads on one or more frame members. The latter scenario may be a shift in the focal line 108 rather than a shift in the position of the tube 110. However, a shift in both the position of the tube 110 and the focal line 108 may be caused by static loads on some or all parts of the frame assembly 100.

Determining the shift in position of the tube 110 relative to the focal line 108 may be based on actual measurements of deflection in or more tube support members (e.g., tube mounts 112) and/or actual measurements of deflection in or more parts of the frame 102 with one or more sensors such as strain gages; actual measurements of the temperature of the surface of the tube 110 and/or the HTF; measurements of the position of the tube 110 and or positions of the reflectors, i.e., the focal line 108, using various imaging techniques such as still or motion photography; measurements of the intensity and size of the radiation band 116 on the tube using light sensors or imaging techniques; and/or, any other displacement sensing, imaging, or thermal measurement techniques.

Determining the shift in position of the tube 110 relative to the focal line 108 may be also be based on predetermined measurement and/or computational data regarding the movement of the tube 110 and/or the movement of the focal line 108 due to static loads. For example, the frame 102 may be cycled through a daily operation and the position of the tube 110 relative to the focal line 108 at several locations along the tube 110 may be measured. Furthermore, such measurements make take into account seasonal variations and/or environmental conditions that may affect the static loads, i.e., deflections in the tube support structure and/or the frame members. In another example, deflections in the tube support structure and/or the frame members may be modeled by computational methods such as finite element analysis. Accordingly, data regarding the deflections in the tube support structure and/or the frame members may be virtually determined with sufficient and/or high accuracy. Determining the shift in position of the tube 110 relative to the focal line 108 may also be determined based on real-time data, historical data, predetermined measurement data and/or other computational data.

According to the method 400, the amount by which to rotate the frame 102 (block 404) may be determined based on data regarding the shift in position of the tube 110 relative to the focal line 108 as described in detail above. Determining the amount by which to rotate the frame 102 may be based on a difference between an actual position of the tube 110 relative to an actual focal line 108, which as described in detail above may be determined in real-time, historical data and/or computational data, and the position of the tube 110 relative to the focal line 108 if the frame 102 is rigid, which may be referred to herein as the rigid position of the tube 110. The processor 300 may then send a command to the drive mechanism 113 to rotate the frame 102 by the determined rotation. For example, if the shift in the position of the tube 110 relative to the focal line is 5° ahead or leading the rigid position of the tube 110, the frame 102 may be rotated by −5° to position the tube 110 relative to the focal line 108 to a near rigid position.

The shift of offset in position of the tube 110 relative to the focal line 108 may be different along the length of the tube 110. According to one example, the method 400 may compute an average of the shift or offset in the position of the tube 110 relative to the focal line 108 along the length of the tube 110 to determine the amount by which to rotate the frame 102. For example, referring to FIG. 1, the shift in the position of the tube 110 may be 1 degree for a first frame 102A, 4 degrees for a second frame 102B, 3 degrees for a third frame 102C and 4 degrees for a fourth frame 102D. According to his example, the frames 102A-D may be rotated by 3 degrees to position the tube 110 to a near rigid position. Other algorithms for determining an overall amount by which to rotate the frame may be used.

The method 400 may be performed by the processor 300 accessing data stored in the data storage device 302 and/or executing one or more program codes stored in the data storage device 302 to operate the drive mechanism 113. For example, the data storage device 302 may include data regarding the rigid position of the tube 110 and the actual position of the tube 110 relative to the focal line 108. During the operation of each frame 102 or a plurality of frames 102 that may be operated by the same control system 115 (e.g., frames 102A-D of FIG. 1), the processor 300 may compute the amount by which to rotate the frame 102 by subtracting the actual position of the tube 110 relative to the focal line 108 from the rigid position of the tube 110, which may be the same as the position of the focal line 108. Based on the result of such computation, the processor 300 may then send a command to the drive mechanism 113 to rotate the frame by the computed amount.

In certain operating conditions, such as during hot summer days, the focusing of sunlight onto the tube 110 may excessively raise the temperature of the tube 110 so as to overheat the tube 110, thereby possibly causing deformation or damage to the tube 110 and/or overheating the HTF, which may adversely affect the viscosity of the HTF. In order to maintain the HTF at an optimum or near optimum temperature while preventing the tube 110 from overheating, the control system 115 can operate in a slightly defocused mode by lag, lead or lead-lag tracking of the sun. In the lag tracking mode or lagging mode, the control system 115 can position a frame assembly 102 slightly lagging from directly facing the sun in order to reduce the number of reflected light rays that strike the tube 110. As a result, the intensity of the focused sunlight is reduced and the HTF is maintained a lower temperature than if the control system 115 tracked the sun without any defocus or lag. However, the lagging offset position may be controlled by the control system 115 so as to maintain the HTF preferred or near preferred temperature.

In the lead tracking mode or leading mode, the control system 115 can position a frame 102 slightly leading ahead of directly facing the sun in order to reduce the number of reflected light rays that strike the tube 110. As a result, the intensity of the focused light rays is reduced and the HTF is maintained at a lower temperature than if the control system 115 tracked the sun without any defocus or lead. However, the leading offset position may be controlled by the control system 115 so as to maintain the HTF at a preferred or near preferred temperature.

In the lead-lag tracking mode or leading-lagging mode, the control system 115 can position a frame 102 slightly leading ahead of directly facing the sun in order to reduce the number of reflected light rays that strike the tube 110. Then, the control system 115 does not move the frame 102 until the position of the sun catches up with the leading position of the frame 102 and passes the leading position such that the frame 102 will be positioned in a lagging mode. The control system 115 then moves the frame 102 to a leading position and this leading-lagging cycle is repeated. As a result, the intensity of the focused light rays is reduced and the HTF is maintained at a lower temperature than if the control system 114 tracked the sun without any defocus or lead-lag. However, the lead-lag cycle may be controlled by the control system 115 so as to maintain the HTF at preferred or near preferred temperature.

Referring back to the radiation band 116 of FIG. 5, the distribution of light rays may not be uniform and have the highest intensity at the center section of the tube 110, which is shown in FIG. 5 as section H, while diminishing from the center section H toward the lateral portions of the tube 110. Referring to FIG. 14, the distribution of the reflected light rays that strike the tube 110 can be represented by a bell-shaped curve BC. The horizontal axis in FIG. 14 represents the width of the radiation band and the vertical axis represents the temperature of the tube 110 along the width of the radiation band 116. Thus, according to the curve BC, the center section H of the tube 110 experiences the highest temperature in the radiation band 116, while the edge portions E experience the lowest temperature of the radiation band 116. It may be preferred to provide a more uniform heat distribution within the radiation band 116, which may result in the transfer of more heat to the fluid carried in the tube, while reducing the peak temperatures that can occur at the center section H of the radiation band 116. A more uniform heat distribution curve of the radiation band 116 may resemble FIG. 15, which shows the peak temperature to be lower, yet more heat being transferred to the tube 110 (i.e., the transferred heat corresponds to the area under the curve BC).

The control system 115 can dither or oscillate the frame assembly 100 about the rotational axis 200 such that the center portion H of the radiation band 116, which is the hottest portion of the radiation band 116, oscillates laterally on the radiation band 116. For example, when the frame 102 is rotated so as to be slightly leading ahead of directly facing the sun, section H of the radiation band 116 is shifted slightly off-center to one side of the tube 110. Accordingly, the curve BC may resemble the curve BC+ of FIG. 16. Conversely, when the frame 102 is rotated so as to be slightly lagging behind from directly facing the sun, section H of the radiation band 116 is slightly shifted off-center to the other side of the tube 110. Accordingly, the curve BC may resemble the curve BC− of FIG. 16. When the control system 115 dithers or oscillates the frame 102 between leading and lagging positions, the oscillations of the curve BC over time as shown by the arrow DT may resemble the curve of FIG. 15. In other words, the peak in the curve BC of FIG. 14 is dithered or oscillated in order for the heat distribution to resemble the curve of FIG. 15. Therefore, the heat transfer to the HTF carried by the tube 110 is increased while reducing the peak localized temperature on the tube 110.

Referring to FIG. 17, a method 500 for controlling the frame 102 when defocusing for overheating and/or dithering for uniform heat distribution is shown. The method 500 includes determining a variable indicative of the temperature of the fluid inside the tube 110 (block 502). The method 500 further includes moving the frame 102 between a focused position and a defocused position wherein sunlight is less focused on the tube 110 by the reflector 106 than the focused position to control the temperature of the heat transfer fluid (block 504). Controlling the temperature of the fluid may be performed by defocusing the frame assembly as described above, or dithering the frame assembly to provide uniform heat distribution on the tube 110.

The variable indicative of the temperature of the HTF may represent the actual temperature of the HTF at a certain location along the tube 110. Alternatively, the variable indicative of the temperature of the HTF may represent the intensity of the radiation band 116 across the width of the radiation band 116, which may be measured by one or more optical sensors as described in detail below. The intensity of the radiation band 116 across the width of the radiation band 116 may correspond to certain temperature of the HTF according historical, tabular, experimental and/or computational data.

If the temperature of the HTF in the tube 110 approaches or exceeds a certain threshold temperature, the control system 115 can rotate the frame 102 to defocus the reflectors 106 and reduce the intensity of the radiation band 116. Accordingly, the temperature of the HTF may be reduced. Further, the control system 115 may continuously dither the frame 102 to provide a uniform heat distribution on the tube 110.

The method 500 may be performed by the processor 300 by accessing data stored in the data storage device 302 and/or executing one or more program codes in the data storage device 302 to operate the drive mechanism 113. For example, the data storage device 302 may include tabular data correlating the intensity of the radiation band 116 to the temperature of the HTF. The processor 300 may receive information from an optical sensor (described in detail below), regarding the intensity of the radiation band 116 at a certain location on the tube 110. If the data received by the processor 300 corresponds to a temperature of the HTF approaching or exceeding a certain threshold, the processor 300 sends a command to the drive mechanism 113 to defocus the reflector or solar assembly to reduce the temperature of the fluid. Furthermore, the processor 300 may send a command to the drive mechanism 113 to dither or oscillate the frame 102 to maintain the temperature of the HTF at a certain level or to evenly distribute the reflected sunlight on the radiation band 116

Referring back to FIG. 4, an exemplary embodiment of the frame assembly 100 is shown. The frame assembly 100 is similar to the frame assembly 100 described above and further includes an optical sensor 122 positioned on the frame assembly 100 so as to face the tube 110. At least one optical sensor 122 may be provided for each frame assembly 100. However, any number of optical sensors 122 can be provided for each frame assembly 100. For example, each frame 102 may include an optical sensor 122. The optical sensor 122 measures the light striking the tube 110. In other words, the optical sensor 122 measures the light that strikes the tube 110 to form the radiation band 116 including longitudinal and lateral gradient information. Accordingly, the curve BC can be constructed with the data obtained from the optical sensor 122.

The data from the optical sensor 122 can be used by the control system 115 to perform the above-described functions of compensating for static loads, compensating for misalignment, defocusing and dithering. Thus, the control system 115 can rotate the frame 102 to shift the peak of the curve BC shown in FIG. 16 to achieve any preferred localized or distributed temperature profile on the tube 110.

The data from the optical sensor 122 can also be used by the control system 115 to track the movement of the sun without running a sun tracking algorithm or receiving information from a sun sensor. When the frame assembly 100 is facing the sun such that the reflectors 106 are focused onto the tube 110, the curve BC resembles the curve shown in FIG. 14. If the frame assembly 100 remains stationary, the curve shifts of center with the movement of the sun. The control system 114 can continuously or discretely receive data from the optical sensor 122 and rotate the frame assembly 100 so as to keep the peak of the curve BC centered. Therefore, by continuously or discretely centering the peak of the curve BC, the control system 115 can track the movement of the sun without resorting to a tracking algorithm, any sun sensors and/or inclinometers. However, a tracking algorithm, a sun sensor and/or an inclinometer can also be used for redundancy, calibration purposes, and/or tracking confirmation.

Although a particular order of actions is described above, these actions may be performed in other temporal sequences. For example, two or more actions described above may be performed sequentially, concurrently, or simultaneously. Alternatively, two or more actions may be performed in reversed order. Further, one or more actions described above may not be performed at all. The apparatus, methods, and articles of manufacture described herein are not limited in this regard.

While the invention has been described in connection with various aspects, it will be understood that the invention is capable of further modifications. This application is intended to cover any variations, uses or adaptation of the invention following, in general, the principles of the invention, and including such departures from the present disclosure as come within the known and customary practice within the art to which the invention pertains. 

What is claimed is:
 1. A method of controlling a solar reflector assembly comprising at least one frame, at least one reflector mounted on the frame, a control system configured to move the frame, and a tube having a central axis and configured to have therein a heat transfer fluid being heated by the reflector focusing sunlight onto a focal line configured to be generally aligned with the central axis, the tube coupled to the frame with at least one tube support, the method comprising: determining an offset between the focal line and the central axis; and moving the frame to move the central axis toward the focal line to reduce the offset.
 2. The method of claim 1, wherein the offset corresponds to a position of the central axis relative to the focal line when at least a portion of the tube support is deflected by a load on the tube support.
 3. The method of claim 1, wherein the offset corresponds to a position of the central axis relative to the focal line when at least a plurality of frame members of the frame is deflected by a load on the frame.
 4. The method of claim 1, wherein the offset corresponds to a position of the central axis relative to the focal line when at least one part of the frame is misaligned relative to another part of the frame.
 5. The method of claim 1, wherein the offset corresponds to a position of the central axis relative to the focal line when the frame is misaligned relative to another frame.
 6. The method of claim 1, wherein determining the offset comprises measuring the position of the central axis relative to the focal line.
 7. The method of claim 1, wherein determining offset comprises computing the position of the central axis relative to the central axis.
 8. The method of claim 1, wherein determining the offset comprises measuring an intensity of light focused on the tube by the reflector with an optical sensor.
 9. The method of claim 1, wherein determining the offset comprises measuring a temperature of the heat transfer fluid.
 10. A solar reflector assembly comprising: at least one frame; at least one reflector mounted on the frame; a tube having a central axis and configured to have therein a heat transfer fluid being heated by the reflector focusing sunlight onto a focal line configured to be generally aligned with the central axis, the tube coupled to the frame with at least one tube support; and a control system configured to move the frame, the control system comprising a processor and a data storage device, wherein the processor is configured to execute a code stored in the data storage device to: determine an offset between the focal line and the central axis; and move the frame to move the central axis toward the focal line to reduce the offset.
 11. The solar reflector assembly of claim 10, wherein the offset corresponds to a position of the central axis relative to the focal line when at least a portion of the tube support is deflected by a load on the tube support.
 12. The solar reflector assembly of claim 10, wherein the offset corresponds to a position of the central axis relative to the focal line when at least a plurality of frame members of the frame is deflected by a load on the frame.
 13. The solar reflector assembly of claim 10, wherein the offset corresponds to a position of the central axis relative to the focal line when at least one part of the frame is misaligned relative to another part of the frame.
 14. The solar reflector assembly of claim 10, wherein the offset corresponds to a position of the central axis relative to the focal line when the frame is misaligned relative to another frame.
 15. The solar reflector assembly of claim 10, wherein determining the offset comprises measuring the position of the central axis relative to the focal line.
 16. The solar reflector assembly of claim 10, wherein determining offset comprises computing the position of the central axis relative to the central axis.
 17. The solar reflector assembly of claim 10, wherein determining the offset comprises measuring an intensity of light focused on the tube by the reflector with an optical sensor.
 18. The solar reflector assembly of claim 10, wherein determining the offset comprises measuring a temperature of the heat transfer fluid.
 19. A method of controlling a solar reflector assembly comprising at least one frame, at least one reflector mounted on the frame, a control system configured to move the frame, and a tube configured to have therein a heat transfer fluid being heated by the reflector focusing sunlight on the tube, the tube coupled to the frame with at least one tube support, the method comprising: determining a variable indicative of a temperature of the heat transfer fluid; and moving the frame between a focused position wherein sunlight is focused on the tube by the reflector and a defocused position wherein sunlight is less focused on the tube by the reflector than the focused position to control the temperature of the heat transfer fluid.
 20. The method of claim 19, wherein moving the frame comprises moving the frame from the focused position to the defocused position to reduce the temperature of the thermal fluid in the tube.
 21. The method of claim 19, wherein moving the frame comprises oscillating the frame between the focused position and the defocused position to provide a generally even heat distribution on the tube.
 22. The method of claim 19, wherein moving the frame comprises oscillating the frame between a first defocused position lagging the focused position and a second defocused position leading the focused position to provide a generally even heat distribution on the tube, wherein the focused position is between the first defocused position and the second defocused position.
 23. The method of claim 19, wherein the variable is determined by measuring the temperature of the heat transfer fluid.
 24. The method of claim 19, wherein the variable is determined by measuring an intensity of light focused on the tube by the reflector with an optical sensor.
 25. A solar reflector assembly comprising: at least one frame; at least one reflector mounted on the frame; a tube configured to have therein a heat transfer fluid being heated by the reflector focusing sunlight on the tube, the tube coupled to the frame with at least one tube support; and a control system configured to move the frame, the control system comprising a processor and a data storage device, wherein the processor is configured to execute a code stored in the data storage device to: determine a variable indicative of a temperature of the heat transfer fluid; and move the frame between a focused position wherein sunlight is focused on the tube by the reflector and a defocused position wherein sunlight is less focused on the tube by the reflector than the focused position to control the temperature of the heat transfer fluid.
 26. The solar reflector assembly of claim 25, wherein to move the frame comprises to move the frame from the focused position to the defocused position to reduce the temperature of the heat transfer fluid.
 27. The solar reflector assembly of claim 25, wherein to move the frame comprises to oscillate the frame between the focused position and the defocused position to provide a generally even heat distribution on the tube.
 28. The solar reflector assembly of claim 25, wherein to move the frame comprises to oscillate the frame between a first defocused position lagging the focused position and a second defocused position leading the focused position to provide a generally even heat distribution on the tube, wherein the focused position is between the first defocused position and the second defocused position.
 29. The solar reflector assembly of claim 25, wherein the variable is determined by measuring the temperature of the thermal fluid in the tube.
 30. The solar reflector assembly of claim 25, wherein the variable is determined by measuring an intensity of light focused on the tube by the reflector with an optical sensor. 